1. Field of the Invention
This invention relates generally to the field of signal receiver and demodulator circuits, and more specifically to circuits adapted to receive and demodulate digitally-modulated, optically transmitted signals. The invention is particularly concerned with threshold circuits that are commonly used in such receiver circuits and with an improved threshold circuit that provides automatic adjustment of the signal threshold level.
2. Statement of Related Art
A common method of transmitting digital data in local area networks and the like involves modulating the intensity of a light source in accordance with the digital data and optically transmitting the modulated light signal over an optical transmission medium, such as a fiber optic cable. Receiver circuits designed to receive and demodulate digitally-modulated, optically-transmitted signals typically include an optical to electrical transducer that converts the optical signal to an electrical signal, a Signal Amplifier that amplifies the converted electrical signal, a threshold circuit that reconstructs the transmitted digitally-encoded signal from the electrical signal, and a demodulator circuit that converts the reconstructed signal into data bit values. Threshold circuits in common use typically generate a constant threshold voltage, compare the amplitude of the converted, amplified, electrical signal with the threshold or decision value, and output a digital signal having one predetermined logic level when the amplitude of the received signal exceeds the threshold, and a second predetermined logic level when it does not.
Such threshold circuits possess several major deficiencies that increase the likelihood of erroneous reconstruction and demodulation of the transmitted signal. For example, in such threshold circuits, the threshold voltage must typically be set so high that under no condition can variations in transmitter or receiver gain or in the level of noise produce a spurious output signal. However, due to the relatively large degree of signal attenuation inherent in commonly-used optical transmission media, the level of the received signal is typically relatively low. In addition, the signal to noise ratio of such signals is typically low. Since such threshold circuits are in effect "desensitized", inaccurate reconstruction and demodulation of these relatively low level, noisy signals may thus occur.
In addition, the threshold level in such threshold circuits is typically set according to the rated gain of the Signal Amplifier and the rated sensitivity of the optical transducer, both of which may vary in operation with time and temperature. Also, such threshold circuits can introduce timing distortion into the reconstructed digitally-encoded signal. This problem occurs because some such circuits react more rapidly to transitions in the amplitude of the electrical signal from below to above the threshold than to transitions from above to below the threshold.
In view of the foregoing deficiencies in the prior art threshold circuits, it is an object of the present invention to provide an improved threshold circuit that automatically adjusts the threshold level to compensate for variations in the amplitude of the received signal due to variations in gain and sensitivity of the various receiver components, variations in noise level, and the like. In so doing, the improved threshold circuit of the invention greatly increases the accuracy of detection, reconstruction, and demodulation of the received signal.
It is another object of the invention to provide an improved threshold circuit that automatically adjusts the signal threshold level to eliminate timing distortion in reconstructed digitally-encoded signals, thereby greatly improving the accuracy of the reconstructed signals.
It is another object of the invention to provide an improved threshold circuit that is suitable for use in typical optical receiver circuits to reproduce digitally-encoded, optically-transmitted signals.